The most common way today for examining the straightness of needles of the above kind, is to roll the needles on a flat plate of metal or stone. By viewing the needle close to and in parallel to the upper surface of the plate while rolling the needle by hand and comparing it with the flat surface, it is tried to locate the position of the needle which exhibit the largest deviation from straightness. When that position is found one has to make a purely subjective estimation of if the viewed deviation is acceptable or not without any aid from any instruments or other facilities. A needle of this kind normally has a width in a range within 8-0.8 mm and the acceptable deviation from straightness may be as small as approximately 0.3 mm. This method is extra difficult to perform while the needle has to be viewed in close parallelism to the plate surface, which makes minor deviations hard to detect.
This situation is not satisfactory in view of the fact that these needles are adapted to perform, in many cases highly critical, diagnostic and therapeutic measures with high precision deep inside tissue. One area of the human body, and probably the most common, where such measures often are performed, is in the brain. After image scanning of the head, a medical-practitioner might find an area which exhibit abnormal changes and it is decided to take a tissue sample of that area. For this reason a hole is drilled through the scull and a biopsy needle is introduced into the target area in the brain. If the biopsy needle should have a curvature, it is an obvious risk that the needle misses the target area and takes a sample from a healthy area instead. Accordingly, the patient may be erroneously declared healthy. Mistakes may also be made, for similar reasons, when performing lesions or stimulations, by means of electrodes, or chemotherapy by means of injection needles of target areas, maybe having to effect that measurements are performed of and injections made into a wrong area if the electrode and the injection needle, respectively, has an unacceptable curvature.
Moreover, when using the needles in practice, they usually are securely attached to a holding structure, preferably a semicircular guide arc fastened to a so called stereotactic frame secured to the scull. For best guidance of the needle, it is common practice to use a guide member attached to the holding structure, which will guide the needle as close as possible to the scull when introducing it into the brain. To examine the straightness of the needle without it being guided by the guide member, will not reflect the actual deviation from straightness under use.
Furthermore, when rolling the needle on the plate surface, the needle has to be touched by hands, which is unfavourable for two reasons. On the one hand, the contact with the needle may affect the straightness of the needle, in case it concerns needles of small dimensions anyway. On the other hand, the contact of the hands with the needle as well as the contact between the needle and the plate surface, will contaminate the needle which excludes the possibility to perform the examination under sterilized conditions just before use of the needle.